A reliable optical process measurement device should withstand occasional high pressures, such as 100 bars, in oil refinery applications or in separation processes used in food industry. An example to be mentioned of such optical measurement devices is a refractometer.
A measurement window or a prism made of an optical material can be installed from the inside of the device, its pressure resistance being thus restricted to pressures below 50 bars. An example of this type of implementation is the process refractometer PR-03 of K-Patents Oy or FI Patent Publication 108259.
An alternative is to install the prism to the process side, as for example in refractometers PR-01 of K-Patents Oy. Due to the prism geometry, the refractometer PR-01 includes two small process seals. Small process seals and sealing grooves are technically difficult to manufacture because of tolerances, for example. Moreover, installation of small seals is laborious for persons installing instrumentation, and includes precision and due to two separate seals the construction is not hygienic.
Further examples of process refractometers that can be mentioned and solutions used in connection with them are the structures disclosed in FI publications 113566 and 118864.